Eh, personally, I see Rifle Assaults as being the ultimate Reserve unit. Run-And-Gun, Plus the range of an assault rifle, means that they can get just about anywhere within your area of deployment. You can have them on your left flank when your right comes under attack, then reposition them to compensate.
Shotgun serves two roles, they can be the vanguard while breaching a UFO, or a troubleshooter, kind of like a more reliable but higher risk version of a Sniper (since you don't have to deal with that unclear LoS/LoE mechanic)